Land seismic data acquisition and processing may be used to generate a profile of the geophysical structure under the surface of the earth. Those trained in the field can use the profile to predict the presence or absence of hydrocarbon accumulations or other geological features. Thus, a high-resolution profile, without error, is frequently preferred over a profile of low-resolution or having a larger margin of error.
Traditionally, a land seismic survey involves the use of seismic sensors and a seismic source. The sensors (e.g., geophones, hydrophones, accelerometers, etc.) are connected to each other and then deployed on or below the surface of the earth. The seismic source is activated and generates seismic waves which propagate through the subsurface until they are reflected and/or refracted by various heterogeneities in the subsurface. The reflected and/or refracted waves propagate to the seismic sensors, where they are recorded. The recorded seismic waves may be used, among other things, for seismic monitoring of producing oil fields if the seismic surveys are repeated over time.
Seismic repeatability, a measure of the fidelity with which a seismic survey is repeated and therefore of its ultimate resolution of changes with time, may be improved when sources and sensors are buried. However, such configuration results in a part of the wave field being reflected in a manner that provides noise. For example, an upwardly directed wave may be transmitted through the weathering layer and reflected at the surface of the earth before observation by the sensor. These surface reflected waves, often called “ghosts,” are affected by the near surface variations and can change over time. The presence of surface reflected waves that fluctuate in time due to temperature and moisture variation in the near-surface may interfere with observations of waves coming from the reservoir or other formations of interest thereby preventing accurate measurement of small reservoir variations.
Techniques for “deghosting” the observations have been developed for both marine and land-based observations. Deghosting sometimes involves placement of vertically spaced sensor arrays (e.g., sensor pairs). By comparing the timing at which various waves are detected at the sensor arrays, it can be determined which of the signals are ghosts and which contains useful seismic data. On land, a wellbore is drilled for each vertically-spaced sensor array. The drilling of a vertical hole for each receiver station with a set of vertically separated receivers per receiver station results in a large drilling effort with high costs and a large environmental imprint.